Surface water, which includes the visible bodies of water like rivers, lakes, and reservoirs, is constantly modified by human infrastructure. Surface water depletion is the reduction in the volume of water within these bodies, often leading to lower flows in rivers or decreased lake levels. This loss is primarily driven by two major human interventions: the creation of dams for large-scale water storage and the construction of diversion systems for water extraction. Dams and diversions fundamentally alter the natural hydrology of a watershed by changing how, when, and where water moves. These mechanisms of water loss, from atmospheric processes to human consumption, combine to reduce the total amount of water available.
Volumetric Loss Through Impoundment and Evaporation
Building a dam transforms a flowing river into a vast, static reservoir, a process called impoundment. This significantly increases the water surface area exposed to the atmosphere. A meandering, relatively narrow river is replaced by a wide, open-water body where water is held for extended periods, accelerating the rate of water loss from the system.
Evaporation is driven by meteorological factors such as solar radiation, air temperature, wind velocity, and humidity. Because a reservoir presents a much larger, stationary surface compared to a flowing river, more water escapes into the atmosphere as vapor. In arid and semi-arid regions, the annual depth of water lost to evaporation can be high, sometimes reaching 7 to 10 feet per year.
This volumetric loss is a consumptive use because it would not occur at the same scale without the creation of the reservoir. For instance, annual evaporation losses from artificial reservoirs in the Western United States alone have been estimated to exceed 11 million acre-feet. This water is physically removed from the local water budget, often rivaling the volume of water extracted for human use in some basins.
Direct Water Removal and Consumption
Surface water is depleted through direct extraction and diversion from rivers, lakes, and reservoirs. Diversion infrastructure, including canals, pipelines, and pumping stations, removes water from the source body for distribution to various users. This process results in an immediate reduction of flow volume in the river or stream.
The majority of surface water diversion worldwide is for agricultural irrigation, which is the largest driver of depletion. Once water is removed and applied to fields, a substantial portion is considered “consumptive use.” Consumptive use is defined as water removed from the source and not returned to the system, primarily occurring through evapotranspiration by crops.
During evapotranspiration, water evaporates from the soil and is transpired through the plants’ leaves, returning to the atmosphere instead of back to the river. Municipal and industrial uses also contribute to consumptive use through processes like manufacturing or evaporation from cooling systems. While some water returns as treated wastewater or runoff, the volume that is truly consumed represents a direct and permanent depletion of the surface water supply.
Cumulative Impact on Downstream Flow and Aquifer Recharge
The combined effects of reservoir evaporation and direct extraction result in a significantly reduced flow volume downstream of the dam or diversion point. This reduction in the natural flow regime has consequences for the entire watershed system. Lower flow magnitudes can lead to the drying up of downstream river sections, reducing the water volume available for wetlands and riparian ecosystems.
The depletion of surface water disrupts the natural connection between rivers and groundwater, known as the surface water-aquifer interaction. Many rivers are hydraulically connected to regional groundwater aquifers, either “gaining” water from the aquifer or “losing” water to it. When river flow is reduced, the water level drops, which reduces the pressure and volume available for aquifer recharge.
In connected systems, the lowering of the surface water level can cause the river to stop replenishing the aquifer, or even lose water to the aquifer at an accelerated rate. This creates a cumulative depletion effect where the loss of surface water volume translates into a reduction in subsurface water resources. The resulting reduction in base flow, which is water supplied by the aquifer to the stream, further exacerbates low-flow conditions, leading to an interconnected depletion of both surface and subsurface water.